1. Field of the Invention
The present invention relates to an optical information recording medium.
2. Related Background Art
In recent years, in the field of an information recording medium, studies on various types of optical information recording media have been carried out. Optical information recording media are being applied to a wide variety of applications as a medium with respect to which information can be recorded and reproduced with high density, in a non-contact mode and at a low cost.
In particular, a Digital Versatile Disk (DVD), which is a large capacity optical information recording medium with respect to which information including not only audio data but also a motion picture such as a movie can be recorded and reproduced, has been widespread in the market.
At present, an optical information recording medium typified by a DVD has a structure in which an information layer is provided on a 1.2 mm thick transparent resin substrate and further a light transmission layer is provided to protect the information layer, or a structure in which an information recording layer is provided on at least one of the surfaces of a 1.2 mm thick transparent resin substrate and they are adhered to each other.
Furthermore, in recent years, in order to achieve high-density recording, an optical information recording medium using a short wavelength laser for recording and reproducing information and an optical information recording medium using an objective lens having a large numerical aperture (NA) are proposed as well (see, for example, JP8 (1996)-235638 A and JP10 (1998)-283683 A).
The size of a recorded mark of an optical information recording medium is proportional to the size of a light spot. Since the size of a recorded mark can be reduced by reducing the size of a light spot, the recording density can be improved. The size of a light spot is determined by λ/NA, where λ denotes a wavelength of irradiated laser beams. That is to say, by increasing NA, high-density recording can be achieved.
There may arise a problem that when NA is increased, a margin with respect to a tilt of the optical information recording medium is reduced. However, this margin can be widened by reducing the thickness of a light transmission layer.
As a method for thinning the thickness of the light transmission layer, there has been proposed a method of adhering a resin film formed by a casting method to the information layer with a UV curable adhesive or an acrylic pressure sensitive adhesive.
FIG. 7 is a cross-sectional view showing a conventional optical information recording medium 10. The optical information recording medium 10 has a configuration in which a substrate 101 with grooves for information recording (not shown) formed on the surface thereof, an information recording layer 102 provided on the surface provided with grooves of the substrate 101, an adhesive layer 103 and a light transmission layer 104 made of a resin film are laminated. As mentioned above, by reducing the thickness of the light transmission layer 104, the margin with respect to the tilt of the optical information recording medium can be increased, thus enabling high-density recording to be realized.
However, when the resin film is used for the light transmission layer and this resin film is adhered to the information layer with an adhesive, there arise the following problems.
An adhesive is a material that requires high adhesion force, however, such a material usually has a porous structure and absorbs water easily. Therefore, when such a material is exposed to high humidity, it absorbs water, which causes a problem that an information layer corrodes or that a tilt occurs in a medium due to a deformation such as a warp. Furthermore, since such an adhesive makes it difficult to diffuse heat effectively, if heat is generated by the irradiation of laser beams at the time of recording/reproducing information, the information layer is damaged by the heat, which may reduce the number of recording/reproducing cycles or reduce reflectance. Furthermore, there arises a problem that a tilt occurs due to a heat deformation of a resin layer.